This invention relates to a light-emitting diode or like semiconductor device made from chemical compounds, particularly those of Groups III-V elements, and to a method of making such a light-emitting device.
A typical conventional compound semiconductor light-emitting device is a lamination of a baseplate of sapphire or silicon carbide or silicon, a buffer layer, an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and a contact layer. Another example, taught by the U.S. Pat. No. 5,008,718 to Fletcher et al., is a lamination of a substrate, an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and a current spreading layer which is also referred to as a transparent window layer.
The light generated at the active layer is emitted through the top-most contact layer or current spreading layer. An anode or top electrical contact is provided in the form of a circular metal pad mounted centrally on the surface of the contact layer or current spreading layer, leaving the rest of the surface area of the contact layer uncovered for light emission. It has also been known to mount a metal pad on the contact layer or current spreading layer via a transparent electrode.
Being impervious to light, the metal pad itself blocks the light from the corresponding part of the active layer. The current flowing in this part of the active layer was wasted, not contributing to the net efficiency of the light emitting device. It has therefore been required for higher efficiency light production to reduce the amount of current flowing in that part of the active layer which is opposed to the pad, and to increase the amount of the current, and make the current distribution more uniform, in the rest of the active layer.
A conventional attempt to meet these requirements is found in Japanese Unexamined Patent Publication No. 2003-197965, which suggests a provision of a metal annulus or rim loosely surrounding, and electrically coupled to, the metal pad on the surface of the contact layer, and of a current spreading layer between active layer and contact layer. Itself impervious to light, however, the annulus reduced by as much as 30 to 40 percent the surface area from which light was emitted, causing a corresponding drop in the efficiency of light production. Additionally, the current spreading layer was not nearly so pervious to light as could be desired, absorbing so much light as to invite a significant diminution in the efficiency of light production.
It might be contemplated to make the electrode itself pervious to light. As heretofore fabricated, however, such an electrode could pass only 70 to 90 percent of the incident light, blocking, in other words, 10 to 30 percent.
The relatively thick current spreading layer proposed by the U.S. patent cited above is also objectionable from the standpoint of maximal light emission. This layer has proved to absorb so much light as to cause a significant decrease in the efficiency of light emission.